1. Field of Invention
This invention relates to a method of hot spot detection and overheating protection of flexible electrical heaters, which have strong metal or carbon containing electrical conductors and insulation with semi-conductive temperature sensitive properties.
2. Description of the Prior Art
Heating elements have extremely wide applications in household items, construction, industrial processes, etc. Their physical characteristics, such as thickness, shape, size, strength, flexibility and other characteristics affect their usability in various applications.
Numerous types of thin and flexible heating elements have been proposed. For example, U.S. Pat. No. 5,861,610 to John Weiss describes the heating wire, which is formed with a first conductor for heat generation and a second conductor for sensing. The first conductor and a second conductor are wound as coaxial spirals with an insulation material electrically isolating two conductors. The two spirals are counter-wound with respect to one another to insure that the second turns across, albeit on separate planes, several times per inch. One of the conductors acts as a heater and another conductor works as a sensing Positive Temperature Coefficient (PTC) wire with predetermined electrical resistance characteristics. The described construction results in a temperature sensing system, which can detect only the average change of resistance in the sensing wire due to elevation of the temperature in the heated product. Therefore, in the event of overheating of a very small surface area (hot spot) of the electric blanket or pad (for example, several square inches), the sensor may fail to detect a minor change of electrical resistance (due to operating resistance tolerance) in the long heating element. In addition, such heating cable does not have inherent Thermal-Cut-Off (TCO) capabilities in the event of malfunction of the controller.
Gerrard (U.S. Pat. No. 6,310,332) describes an elongated heating element for an electric blanket comprising a first conductor means to provide heat for the blanket and extending lengthwise of the element, a second conductor means extending lengthwise of the element, and a meltdown layer between the first and second conductor means which is selected, designed and constructed or otherwise formed so as to display a negative temperature coefficient (NTC), and including an electronic controller set to detect a change in the resistance of the meltdown layer to provide a means of changing the power supply to the first conductor means (providing heat to the blanket), to prevent destruction of the melt down layer. The element further includes a meltdown detection circuit for detecting meltdown of the meltdown layer and for terminating power to the first conductor means in the event that the control means fails and the meltdown layer heats up to a pre-determined degree. The disadvantage of this construction is that the final safety of the blanket relies on a complex NTC/meltdown detection system located in the controller. In the event of controller failure, or significant delays in the detection of the NTC layer meltdown, severe scorching of the heating product or a fire can occur. The Gerrard heating system always requires separate sensing PTC wire, attached to the controller to detect overheating or hot spots. Such passive PTC sensing conductor needs an additional pair of lead wires going from the heater to the sensing control system, which increases weight, size and cost of the heating systems.
Another disadvantage of Gerrard's invention is that its control system utilizes a half-wave power cycle for heating and another half-wave power cycle for meltdown stroke detection in order to provide proper heating output and meltdown protection. Therefore, the heating wire has to be twice as thick as systems utilizing a full-wave power output. This feature becomes especially challenging for 120V and other lower voltage heating systems, compared to traditional European 240V systems. Increased thickness of the heating wire leads to: (a) increased cost of the heating conductor; (b) increased overall size of the heating element and (b) increased heating wire susceptibility to breaking due to reduced flexibility.
Kochman (U.S. Pat. No. 6,713,733) describes a soft and flexible heater which utilizes electrically conductive threads or fibers as heating media. The conductive fibers are encapsulated by negative temperature coefficient (NTC) material, forming temperature sensing heating cables. The heater may contain continuous positive temperature coefficient (PTC) temperature sensors to precisely control the temperature in the heater. The disadvantage of this system is that it requires at least two independent conductors connected to the control system. The first conductor acts as a heating means and the second conductor acts as a heat detection conductor. The NTC hot spot detection system becomes less sensitive with increase of the length of the cable. The heating means and heat detection conductor require separate connections by lead wires to the controller. The electronics which detect overheating use a signal (drop of potential) which transfers to the electronic controllers through heat sensing conductors. The addition of heat sensing conductors for signal transfer and the addition of extra lead wires, results in increased size of the heating cable and lead wire cord, thereby reducing their flexibility and increasing their weight and cost.
The present invention seeks to alleviate the drawbacks of the prior art and describes the novel method of hot spot detection, overheat protection and the fabrication of a heater comprising at least one of the following heating means: metal wires, metal fibers, metal coated, carbon containing or carbon coated threads/fibers, which results in a flexible, strong, heating element core. A preferred embodiment of the invention consists of utilizing electrically conductive textile threads/fibers having an inherent Thermal Cut Off (TCO) function to prevent overheating and/or fire hazard. However, the proposed heaters preferably contain metal conductors or combination of metal wires and conductive textile fibers. The system utilizes an NTC sensing layer for hot spot detection, which does not require having low-temperature meltdown characteristics. The heaters described in this invention may also comprise a continuous temperature PTC sensor to precisely control heating power output in the heating product. The system comprises a current leakage conductor and an electronic or electromechanical device for detecting and comparing the current imbalance in the heater. One of such devices may contain Ground Fault Circuit Interrupter (GFCI), which is also commonly known as “Earth Leakage Circuit Breaker” (ELCB)) to detect current imbalance in the heater due to current leakage through the NTC sensing layer from the heating means to the current leakage conductor. Simultaneously, the same GFCI or other current leakage detecting device can protect the heating cable from mechanical intrusion in the heating cable. Such mechanical intrusion may be in the form of moisture (water) penetration, heating element damage or direct electrical contact between the heating means and the ground conductor due to metallic intrusion inside the heating cable.